1. Field of the Invention
The present invention relates to proportional pump systems for metering viscous fluids and, more particularly, to an improved proportional pumping system that provides for the mixing and dispensing of two or more fluids in a prescribed mix ratio until a batch or steady state rate is attained, regardless of changes in system fluid characteristics.
2. Description of the Background
The problem of mixing and dispensing fluids while maintaining precise volumetric ratios of one fluid to another throughout the mixing/dispensing process arises in the context of many industrial applications. Specifically, there are applications in the fields of resin preparation and dispensing, soft drink dispensing, paint formulation, and liquid chromatography, to name only a few. The required proportions in mixing systems vary greatly depending upon the particular industrial application, as does the need for accuracy. In current viscous fluid mixing apparatus, particular variations in mix accuracy can range anywhere from 0.5% to 1.0%, depending upon the industrial application. Where large volumes of product are at stake, any lapses in mix accuracy can result in large amounts of wasted product and high economic losses. Whether the industrial application is for the mixing of food products, construction products, pharmaceutical products, or any other industrial mixing requirement, maintaining accurate proportions is an economic necessity.
There are a variety of commercial mixing systems, and many factors should be considered when selecting the best for a particular application. For example, the mixing system should be matched to the expected range of flow rates, e.g., is the system configured for filling a five gallon pail or a 5,000 gallon tank? The system should also be able to adjust flow rates when new or unusual conditions are encountered in the pumping system. Often times, a change in influent properties will change the effluent quality of the mix. Because the overall performance of a mixing system is ordinarily measured in terms of the effluent quality, which in turn must be consistent with the user's effluent discharge requirements, it is imperative that mixing systems be able to quickly adapt to system changes so that the required effluent quality may be maintained.
Proportional pumping systems have long attempted to provide for the automatic adjustment of fluid flows in order to maintain a proper proportional flow between the fluids being mixed. Unfortunately, the prior art devices have been unable to achieve the desired level of accuracy in precision mixing and dispensing of highly viscous fluids at large flow rates. One recurring shortcoming lies in achieving the necessary accuracy in flow measurements for highly viscous fluids, due to variations in the absolute and relative pressures of the various system fluids during the dispensing operation. Changes in fluid pressure alter the fluid flow rates individually and with respect to one another, thus changing the ratio of fluids in the final mix. Some fluid mixing systems have attempted to compensate for these pressure changes through manual adjustment of varied flow control apparatus, requiring an operator to vigilantly guard the various flows so that manual adjustments may be made as needed. Other prior art devices have used flow restrictors, such as a flow washer or a metering pin, to control flow rates over limited ranges of pressures. However, flow restrictors do nothing to account for temperature and viscosity variations in the fluid. Still other prior art devices have incorporated a microprocessor to measure flow rates and adjust flow rates at predetermined intervals to maintain a desired proportional flow. These systems attempt to maintain proportional flow rates by measuring and comparing the pressures and velocities of the various flow lines, and use that data to control pumps, valves, or other system components to maintain proportional flows. Unfortunately, such systems are complex, requiring multiple valves, meters, piping, and pumps to measure particular properties of each flow line at various points in order to calculate the proportional flow rates. Such components further increase the disturbance in the flow lines and are prone to damage in systems pumping corrosive chemicals.
For example, U.S. Pat. No. 5,490,726 to Davis et al. discloses a device for controlling the relative ratio of resin to catalyst by adjusting the catalyst (secondary fluid) pressure as a function of the pressure at which the resin (primary fluid) is supplied to dispensers, and by adjusting the duty cycle of a catalyst valve within the catalyst flow path.
U.S. Pat. No. 5,360,320 to Jameson et al. discloses a proportional pumping system for supplying two solvents into a mixer. Pressures are measured in each fluid conduit to generate first and second signals that are proportional to those pressures. Those signals are then compared to the programmed pressure to give an error signal which in turn is multiplied by concentration signals from a programmer to control each pumping rate.
U.S. Pat. No. 5,143,257 to Austin et al. discloses a system for proportional fluid dispensing comprising a solenoid operated pump which discharges controlled amounts of secondary liquid into a main flow. A flow meter sends a signal relating to the main flow rate. An optical pressure sensor sends a signal showing the static pressure of the main flow. Logic circuits combine the flow rate/pressure signals for varying pump operation to maintain the appropriate ratio by controlling the primary flow rate.
U.S. Pat. No. 5,092,739 to Gill discloses an electronic control for a pump comprising input means which receives signals/values for quantities that determine the flow rate to be supplied by a pump. The device has an electronic memory which holds preset values for those quantities and contains stored control signals corresponding to the desired flow rate. The device has output means which supply the appropriate control signal to a pump. The device input consists of data relating to the overall fluid supply job, and the device output consists of the appropriate flow rate calculation which in turn generates a pump control signal.
U.S. Pat. No. 5,033,644 to Tentler discloses a device for dispensing varying viscosity fluids in proper ratios whereby flow rates of the fluids are sensed by flowmeters, and a pressure-sensing feedback circuit allows the proper proportioning of flows. The proportioning of fluids may be changed either by specific user action or by programming a new desired ratio.
It is noteworthy that the above-referenced patents monitor only one or two fluid properties such as pressure, and adjust the catalyst flow path accordingly based on percentage change. For instance, the '644 patent accounts for variations in the flow rate by changes in fluid parameters, e.g., slip (pressure drop) or pressure differentials (increase heads, changes in temperature, or in-line restrictions). This is inadequate because the secondary fluid will correspond to the change and either increase or decrease by percentage according to preset proportions. Changes to system fluid viscosity are a significant source of error in maintaining a proper proportion of fluids in a final product. Highly viscous fluids, such as resins, retain a high dependence on temperature. Because ordinary mixing and dispensing procedures involve numerous sources of temperature change, system fluids which are temperature sensitive will regularly experience a change in viscosity. Such a change in viscosity will be accompanied by a change in flow rate if the same amount of pumping pressure is applied to the fluid.
Thus, there remains a need to be able to instantaneously modify the system flow rates when a change in viscosity is realized in order to maintain the desired proportions of fluids. In addition, the flow rate of the secondary fluid should be controlled based on all critical fluid flow properties such that a proper proportional flow may be maintained between any number of various fluids, and should provide for changes in viscosity of a system fluid.
The foregoing is particularly true of the Cured In Place Pipe (CIPP) industry in which piping systems are repaired through the application of resin compounds to damaged pipe surfaces while the pipes remain buried underground. The steps of the CIPP process are generally as follows:
Step 1: Resin saturated liner is installed in an existing pipe through a manhole or the like.
Step 2: Water is used to fill and invert the liner within the pipe and is continually added to maintain a constant pressure. The water pressure keeps the liner pressed tightly against the walls of the pipe.
Step 3: The water in the pipe is circulated through a heat exchanger where it is heated and returned to the pipe. The hot water cures the thermosetting resin, causing it to harden into a structurally sound, jointless pipe-within-a-pipe.
Step 4: Water pressure is released and the liner is trimmed and cut with a remote control cutting device or man-entry techniques. The lined pipe is ready for immediate use with no excavation whatsoever.
A variety of resins may be used to repair pipes during the above-described CIPP process. The application for impregnation consists mainly of polyesters or vinyl esters and utilize a thin or less viscous catalyst. A typical ratio of resin to catalyst to secondary catalyst or promoter during the CIPP process is around 100:1:0.5 by weight. Other resins, such as epoxies, require the use of curing agents which range from low viscosity to high viscosity. For the application of epoxies, ratios of resin to curing agent may range from 2:1 to 100:4 by weight. Thus, an epoxy system may have a viscous primary fluid resin and either a slightly viscous secondary fluid curing agent or a highly viscous secondary fluid curing agent. The prior art devices have not been able to provide a versatile system that could accurately control the flow characteristics of such systems having numerous viscous fluids.
These variations between system fluid viscosities aggravate the problem of dispensing both resin and catalyst together while maintaining a precisely prescribed mix ratio over a range of temperatures, and over a corresponding range of fluid viscosities. Shell Oil Company has published a document entitled "Epon Resin Systems for In-Place Pipe Rehabilitation" which gives an excellent overview of the CIPP industry, including fluid characteristics of epoxies and epoxy curing agents, and industry specific needs and requirements, and a CIPP application newsletter and charts explaining the viscosity/temperature relationship for resins used in the CIPP industry. As shown in that publication, the highly viscous resins used in the CIPP industry show a logarithmic relationship to temperature, wherein a small reduction in operating temperature below 75.degree. F. will result in a large increase in fluid viscosity. Often in the CIPP application, the temperature of the resin during system operation is maintained at approximately 70.degree. F. At this temperature, any slight change in temperature will result in a large change in fluid viscosity, and therefore in fluid flow rate, thus aggravating the problem of attempting to accurately regulate and control proportional amounts of fluid being dispensed. These variations in fluid viscosity in turn cause variations in the signal outputs of prior viscosity-sensitive flow metering devices, which likewise results in a correspondingly undesirable error in mixing proportions.
A particular source of temperature variation exists in the form of the shear forces exerted on a fluid by the pump. The shear forces applied to the system fluid depend on the particular pump configuration and the differential pressure applied to the fluid as it travels through the pump. Shear forces are evidenced by the addition of heat to a viscous fluid, as heat is generated from frictional forces within the fluid as it escapes its intended flow path through the pump (i.e., pump slip). The heat transfer causes temperature fluctuations which again alter the fluid viscosity, thus effecting the accuracy of flow monitoring and control. Unfortunately, the prior art devices do not account for these temperature and viscosity variations, and are therefore unable to achieve the necessary level of accuracy for precise fluid mixing applications.
Accordingly, it would be a great advantage to provide a proportional fluid pumping and dispensing system which allows the accurate proportional mixing of fluids having differing viscosities regardless of changes in pressure or viscosity in the fluids. It would be another advantage to provide such a system that would function accurately regardless of temperature fluctuations in the system fluids which would in turn effect fluid viscosities. It would be another advantage to provide such a system which could be readily operated by unskilled personnel who could input the necessary fluid parameters for any fluid and operate the system without the need for continual inspection or manual adjustment. It would be yet another advantage to provide such a system that is readily adaptable to particular applications, allowing for the mixing of any number of fluids of any viscosity. It would be yet another advantage to obtain large flow rates at low RPM's that would lessen shear forces, and thus the addition of heat, and product thinning would be minimized.